For me it's the Sega-CD. Just picked one up and looking forward to some spare time to weed through the game library. It's like the awkward teenager of gaming, still connected to the 16-bit Mega Drive, but not quite a fully fledged new system with truly better capabilities, and game design reflected that. So many cheesy games, I can't wait.

I own none of those.

So VC games on my Wii / Wii U.

Occasional GC as well.

I have to play some GC games, never finished Baten Kaitos, and want to play Skies of Arcadia.

I never stop playing F-Zero GX...I'm playing it with my mind right now. 

Kinda off topic: is the Lost Levels worth it? Never played it before and it's available as a reward on Club Nintendo, but I'm not sure if I should get it or not.

Allll of them? I've been giving my Dreamcast some love lately. I did just get a TurboGrafx 16 a few months ago and havn't really touched it yet.

This year, I bought a Famicom and a Super Famicom in spring, and I look forward to play more games on those two systems. I have to watch my money at the moment, which is why I own few games for them right now.
I also just recently got a SNES and 6 PlayStations, which I'll probably play for a considerable amount of time next year.
I also went back to my GameCube/GBA days and bought some new games for those systems. Since my WiiU and PS3 are at my parents' and not in my apartment, those two (GC/GBA) will see heavy use as well. I also got a PSP, which isn't exactly retro, but most of it's games are SNES/PS-Remakes, so I figured I could list it as well.

Honestly, most of my gaming is done on "retro" consoles at the moment. If not on that, I usually play on 3DS/PSVita right now, and sadly, I rarely get to play on my recent home consoles. Even though I still buy games for them (especially WiiU, my PS3 is really only used for Sonic games, Valkyria Chronicles and Persona 4 Arena).

PS2!

It's retro now

N64 all day.

TG-16 for life.